The overall objective of this ongoing study is to research the interaction among cellular compartments in mammary gland and to determine if and how their activities are coordinated. To do this, we have examined the relationship between the mitochondria and the cytosol in support of lipogenesis. To this end, mitochondrial utilization and production of cytosolic substrates and the rate of anionic transport across the mitochondrial membrane have been determined. From an analysis of our results, we have concluded that the cytosolic and mitochondrial compartments are indeed coordinated and that the interaction between them is modulated during the physiological transitions of the gland. We have not, however, uncovered the site(s) of this modulation. Anionic transport and the rate and manner of substrate utilization by mitochondria have been ruled out. Several pieces of evidence, concentration of citrate and malate in the mitochondria and the binding of cytosolic enzymes to mitochondria indicate a role for specific enzymes in each compartment acting as the implementors of this coordination. Direct evidence for this possibility is being sought. Identification of the messengers for directing this enzymatic activity remains for future investigation.